86 INTRODUCTION TO GENERAL PHYSIOLOGY 



since there must be as many carbon dioxide molecules leaving the 

 surface of the liquid in a given time as there are molecules entering 

 it; otherwise there could be no equilibrium. But although the tension 

 must be the same whatever the liquid taken, the actual amount of 

 carbon dioxide dissolved may vary greatly. Thus, water dissolves 

 more than a strong salt solution docs. Further, the amount of 

 different gases dissolved by water varies greatly at the same 

 tension. 



If now we take a solution of haemoglobin and expose it to the 

 air, that is, to 160 mm. tension of oxygen, we find that it takes up 

 a much larger amount of oxygen than water does. Next, expose 

 it to a vacuum or an atmosphere of pure nitrogen, that is, to a zero 

 tension of oxygen ; the oxygen which it contains is given off 

 again. What happens, however, if we expose it to a tension of 

 oxygen of one-eighth of that in the atmosphere, namely, to 20 

 mm.? We find that it takes up oxygen, but less than at 160 mm. 

 Moreover, at all tensions between o and 100 mm. the amount of 

 oxygen taken up has a definite value; and, if a graph be made, 

 it will be seen that these values are not directly proportional to 

 the tension, but the curve rises more steeply at the lower tensions, 

 so that very little more is taken up at 100 mm. than at 80 mm., 

 while above 100 mm. haemoglobin is practically saturated with 

 oxygen, a further rise in tension resulting in no further amount 

 being taken up. 



Although this union of oxygen with haemoglobin is usually 

 looked upon as a chemical compound, it must not be overlooked 

 that there are difficulties in this view, and that it is held by some 

 that the case is more analogous to the adsorption of gases by 

 surfaces such as that of charcoal, which played so great a part 

 in the protection from poison gases in the War. The question 

 is not yet decided, and more complete discussion^ may be read 

 elsewhere (P., pp. 613-625). But there are some facts of interest 

 that may be mentioned briefly here on account of their interest 

 and importance. 



We saw above that when haemoglobin has taken up a certain 

 amount of oxygen, any further rise in tension does not result in 

 any more being taken up. In speaking of the chemical nature 

 of haemoglobin, it was pointed out that each molecule contains one 

 atom of iron. Hence, by comparing the amount of oxygen taken 

 up in saturation with the amount of iron, it is possible to find out 

 the number of molecules of oxygen that unite with a molecule 

 of haemoglobin. It is found to be precisely one molecule. This 

 is a fact strongly indicating a definite chemical compound. But 

 a difficulty arises at once as to the nature of the compounds 

 present when the haemoglobin is only partially saturated. It 



